Methanogens are members ofthe human microbiome and the only known source of biogenic methane. Residing in the third domain of life, Archaea, they are strict anaerobes that have evolved systems to combat periodic oxygen exposure. There is a significant gap in our knowledge of how methanogens mount a transcriptional response that ultimatley leads to cell survival during periods of oxygen stress. The overall goal of this project is to understand the role of MsvR in regulating the oxidative stress response in methanogenic Archaea through structure/function analysis. MsvR is a newly described transcripfional regulator present in select methanogens including those residing within the human intestine and oral cavity. Early characterization of MsvR has led to the development ofthe following hypotheses: HI) multiple amino acid residues in MsvR play a role in its function as a transcriptional regulator, (H2) oxidized and reduced MsvR play distinct yet integral roles in the regulation of expression of at least two promoters, and (H3) MsvR undergoes various conformational changes when shifting between the oxidized and reduced states that impact its function. To address these hypotheses the following specific aims are proposed: (Aim 1) Determine the 3D structure of MsvR by X-ray crystallography, (Aim 2) Identify additional amino acid residues or combinations thereof in MsvR necessary for function, (Aim 3) Identification of disulfide bonds formed upon oxidative stress and the role of oxidized vs. reduced MsvR in transcription regulation, (Aim 4) Identification of conformational changes in MsvR.